1. Field
The present disclosure generally relates to optical networks. More specifically, the present disclosure relates to a multi-chip module (MCM) that includes integrated circuits that communicate via an optical network, dedicated point-to-point optical links and opportunistic stealing of communication bandwidth.
2. Related Art
Wavelength division multiplexing (WDM), which allows a single optical connection to carry multiple optical links or channels, can provide: very high bit-rates, very high bandwidth densities and very low power consumption. As a consequence, researchers are investigating the use of WDM to facilitate inter-chip communication. For example, in one proposed architecture chips (which are sometimes referred to as ‘sites’) in an array (which is sometimes referred to as a multi-chip module or MCM, or a ‘macrochip’) are coupled together by an optical network that includes optical interconnects (such as silicon optical waveguides).
In order to use photonic technology in interconnect applications, an efficient design is needed for the optical network. In particular, the optical network typically needs to provide: a high total peak bandwidth; a high bandwidth for each logical connection between any two sites in the array; low arbitration and connection setup overheads; low power consumption; and bandwidth reconfigurability.
A variety of optical network topologies having different characteristics and contention scenarios have been proposed to address these challenges in interconnect applications. One existing optical network topology, a static WDM point-to-point optical network, is shown in FIG. 1. In this optical network topology, an array of integrated circuits or chips 0-3 (which are each located at a ‘site’ in the array) are coupled by silicon optical waveguides using two carrier wavelengths (represented by the solid and dotted arrows). Note that the optical network in FIG. 1 is a fully connected point-to-point optical network. In particular, each site has a dedicated optical link or channel to every other site. Links to all the sites in a column of the array (which are conveyed by different carrier wavelengths output by non-tunable light sources) may be multiplexed using WDM onto a single waveguide that runs from the source site and visits each site in the column, where a carrier wavelength-selective ‘drop filter’ redirects one of the multiplexed carrier wavelengths to a destination site (in this case, the drop filters in row 1 pick off the first carrier wavelength, and the drop filters in row 2 pick off the second carrier wavelength, so the carrier wavelength is used for routing). As illustrated by the bold line, in FIG. 1 chip 0 communicates with chips 1 and 3.
A key property of this optical network is the lack of arbitration overhead, which allows low minimum latency and high peak utilization for uniform traffic patterns. Furthermore, this optical network uses no switching elements, which results in low optical power loss in the optical waveguides. However, the bandwidth in the optical waveguides is statically allocated, which constrains the available bandwidth between any two sites. For example, in a macrochip that includes 64 chips arranged in an 8×8 array, with a peak system bandwidth of 20 TB/s, a total transmit bandwidth of 320 GB/s and a total receive bandwidth of 320 GB/s for each site, the bandwidth between any two sites is 5 GB/s, because each site has 64 outgoing optical waveguides so that each optical waveguide only has 1/64th of the total site bandwidth. This constraint can lead to low performance for workloads that heavily stress a subset of the optical waveguides.
Alternatively, an optical network can enable sharing of optical links, for example, by combining the carrier wavelengths of multiple optical links to form a single logically shared optical link. Optical networks based on sharing can potentially provide higher site-to-site bandwidths compared to a point-to-point optical network, albeit at the cost of arbitration delays in accessing the shared optical link. However, in optical networks there is typically another significant cost associated with sharing: increased power consumption.
Usually, optical networks are static power dominated, including the optical power (laser) and the ring-resonator-modulator tuning power. A continuous-wave laser source is always active regardless of whether the optical link is idle or busy. Moreover, the optical power required for an optical link is a function of the number of devices and the optical power loss per device on that optical link. Because shared optical-network architectures often use additional devices (for example, additional ring-resonator modulators and switches) to enable sharing, the power loss can be significantly larger. The use of additional ring-resonator modulators can also result in larger ring-resonator-modulator tuning power relative to a point-to-point optical network because the ring-resonator modulators often need to be thermally tuned and maintained at the proper operating temperature at all times. Therefore, while sharing designs typically offers higher site-to-site bandwidths, this often comes at the cost of increased static power consumption.
Hence, what is needed is an MCM with an optical network that does not suffer from the above-described problems.